Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities formed as a result of the oxidation of paraxylene.
Terephthalic acid (TPA) is an intermediate in the production of polyesters for plastics and fiber applications. Commercial processes for the manufacture of TPA are often based on the heavy-metal catalyzed oxidation of p-xylene, generally with a bromide promoter in an acetic acid solvent. Due to the limited solubility of TPA in acetic acid under practical oxidation conditions, a slurry of TPA crystals is usually formed in the oxidation reactor. Typically, the TPA oxidizer slurry is withdrawn from the reactor and TPA solids are separated from the oxidizer mother liquor using conventional solid-liquid separation techniques. The oxidizer mother liquor, which contains most of the catalyst and promoter used in the process, is recycled to the oxidation reactor. Aside from the catalyst and promoter, the oxidizer mother liquor stream also contains dissolved TPA and many by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions formed as a result of the oxidation of p-xylene to terephthalic acid. Patents disclosing the production of terephthalic acid such as U.S. Pat. No. 4,158,738 and U.S. Pat. No. 3,996,271 are hereby incorporated by reference in their entirety to the extent that they do not contradict statements herein.
The TPA solids undergo a solid-liquid separation wherein fresh solvent is utilitized to displace a major portion of the liquid component of the oxidizer mother liquor.
Many of the impurities in the oxidizer mother liquor stream that are recycled are relatively inert to further oxidation. Such impurities include, for example, isophthalic acid, phthalic acid and trimellitic acid. Impurities, which may undergo further oxidation are also present, such as, for example, 4-carboxybenzaldehyde, p-toluic acid and p-tolualdehyde. Oxidation inert impurities tend to accumulate in the oxidizer mother liquor upon recycle. The concentration of these inert impurities will increase in the oxidizer mother liquor until an equilibrium is reached whereby the rate of removal of each impurity via the TPA product balances with the rate of formation and the rate of addition to the oxidation process. The normal level of impurities in commercial crude TPA makes it unsuitable for direct use in most polymer applications.
Conventionally, crude TPA has been purified either by conversion a dimethyl ester or by dissolution in water with subsequent hydrogenation over standard hydrogenation catalysts. More recently, secondary oxidative treatments have been used to produce polymer-grade TPA. It is desirable to minimize the concentration of impurities in the mother liquor and thereby facilitate subsequent purification of TPA. In some cases, it is not possible to produce a purified, polymer-grade TPA unless some means for removing impurities from the oxidizer mother liquor stream is utilized.
One technique for impurity removal from a recycle stream commonly used in the chemical processing industry is to draw out or “purge” some portion of the oxidation mother liquor recycle stream. Typically, the purge stream is simply disposed of or, if economically justified, subjected to various treatments to remove undesired impurities while recovering valuable components. One example is U.S. Pat. No. 4,939,297 herein incorporated by reference in their entirety to the extent that they do not contradict statements herein. The amount of purge required for control of impurities is process-dependent; however, a purge amount equal to 10-40% of the total oxidizer mother liquor stream is usually sufficient to produce TPA adequate as feedstock for commercial polymer manufacture. In the production of TPA, the percentage purge of the oxidizer mother liquor stream purge necessary to maintain acceptable impurity concentrations, coupled with the economic value of the metal catalyst and solvent components in the oxidizer purge stream, make simple disposal of the oxidizer purge stream economically unattractive. Thus, there is a need for a process that recovers essentially all of the valuable metal catalysts and acetic acid contained in the oxidizer purge stream while removing a major portion of the impurities present in the oxidizer purge stream. The metal catalyst can be recovered in an active form suitable for reuse by direct recycling to the p-xylene oxidation step.
A number patents teach a terephthalic acid process comprising a purge process comprising concentration, filtration, followed by extraction.
In such purge processes, oxidation by-products exit the process in two locations. Oxidation by-products that come out of solution in the purge concentration zone due to solvent loss are isolated and washed in a subsequent filtration zone to generate an oxidation by-product rich wet cake which can exit the process. Also, those oxidation by-products that remain in solution after the concentration zone pass through the filtration zone in the liquid mother liquor which is then routed to an extraction and distillation zone where oxidation by-products are concentrated into a waste sludge stream suitable for exiting the process. It is desirable to minimize the catalyst content in the oxidation by-product wet cake generated in the filtration zone as well as the oxidation by-product sludge stream generated in the extraction and distillation zone. Any catalyst present in these two streams will not be routed back to the p-xylene oxidation reactor and will therefore represent a loss of catalyst in the terephthalic acid process.
Water is added to the purge filter mother liquor in a mix zone just prior to extraction, generating a stream that is a mixture of mother liquor and water. The purpose of adding water to the purge filter mother liquor just prior to the extraction zone is to adjust the water:acid ratio for acceptable performance in the extraction zone. We have discovered that splitting this water stream and routing a portion of it to a mix zone immediately before the purge filter where the water is mixed with concentrated filter feed just prior to filtration has a material impact on decreasing the cobalt content in the purge filter wet cake generated in the purge filtration zone. It should be noted that splitting the water stream normally added just prior to extraction and routing a portion of the water stream to the filter feed just prior to filtration does not impact the water:acid ratio of the purge filter mother liquor and water mixture routed to extraction because any water added prior to filtration simply passes through the filtration zone with the filtration mother liquor.